This invention relates to a device for an electrochemical battery which responds to an increase in temperature to break an electrical circuit within a cell, thereby stopping the flow of current and preventing an increase in pressure within the cell to an undesirable level.
Batteries are used to generate electrical energy to operate electronic devices. When batteries are misused or otherwise subjected to abusive conditions, the energy they are capable of producing can create potentially dangerous conditions. For example, exposure to high temperatures can create high internal pressures, and if the internal pressure becomes too great the battery housing can be forced open, and housing and internal components can be forcefully ejected. A battery with a metal with a relatively low melting point, such as a lithium battery, can also be heated sufficiently to melt that metal, causing an internal short circuit and runaway exothermic reactions. Exposure to abnormal or abusive electrical conditions can also generate a large amount of heat.
To prevent rupturing of the battery container and forceful ejection of battery sealing components, batteries often have pressure relief mechanisms, or vents, that will open to relieve the internal pressure at a lower level. However, this does not necessarily prevent the release of potentially dangerous fluids (e.g., corrosive electrolytes) or prevent the continued generation of heat within the cells. An example of a battery with a pressure relief vent in the cell container is disclosed in U.S. Patent Publication No. 2004/0157115 A1 and U.S. Pat. Nos. 6,348,281, 6,346,342 and 4,803,136, all incorporated herein by reference.
To prevent unnecessary opening of the pressure vents, fuses have been incorporated into some batteries, particularly higher energy batteries (e.g., rechargeable alkaline batteries such as nickel/cadmium batteries, primary and rechargeable lithium batteries with a variety of active positive electrode materials, and rechargeable lithium ion batteries). However, fuses permanently break the electrical circuit do not allow the battery to be used once the abusive condition has been removed, even if the battery has not been damaged. Examples of batteries with fuses also incorporated into the cells are disclosed in U.S. Pat. Nos. 4,879,187 and 4,188,460, which are incorporated herein by reference.
As an alternative to a fuse, other types of current interrupters have been used. Some of these respond to internal pressure and some respond to heat. The thermally responsive current interrupters can make use of bimetallic or shape memory alloy components that change shape when their temperatures exceed predetermined values, and some also incorporate a diode, such as a Zener, Schottky, or power rectifier diode, to generate additional heat if the current flow exceeds a desired maximum. Some current interrupters permanently break the electrical circuit, while some are reversible. Examples of batteries with such current interrupters are disclosed in the following U.S. patents, all of which are incorporated herein by reference: U.S. Pat. Nos. 6,037,071; 5,998,051; 5,766,793; 5,766,790 and 5,747,187. Additional examples are also disclosed in Unexamined Japanese Patent Publication No. 05-205,727. The batteries disclosed in these references have one or more disadvantages. They may require additional components, adding to the battery cost, complicating the manufacturing process, and often increasing the internal resistance, thereby adversely affecting battery performance, particularly under heavy discharge conditions (e.g., low resistance, high current and high power). Some do not include a pressure relief vent, so a separate vent is required. In some the operation of the current interrupter coincides with the operation of the pressure relief vent, so breaking the internal circuit does not serve to prevent venting of potential harmful fluids.
Some batteries have used positive temperature coefficient (PTC) devices, either instead of or in combination with a fuse or reversible circuit breaking device. When the flow of current exceeds a threshold limit in a PTC device, or the PTC device otherwise exceeds a threshold temperature, the resistance of the PTC device increases rapidly to reduce the flow of current to a very low level. This provides protection against electrical abuses such as external short circuits, overcharging and forced discharge. However, it does not completely break the electrical circuit between the positive and negative electrodes. The addition of a PTC device to a battery also has disadvantages similar to those of reversible circuit interrupters: increased cost, manufacturing complexity, and internal resistance.
An example of a cell according to the prior art and containing a pressure relief vent and a PTC is a primary Li/FeS2 cell with a nonaqueous electrolyte, such as the cell shown in FIG. 1. Cell 10 is an FR6 type cylindrical Li/FeS2 battery cell. Cell 10 has a housing that includes a container in the form of a can 12 with a closed bottom and an open top end that is closed with an internal cell cover, or sealing plate, 14 and a gasket 16. The can 12 has a bead or reduced diameter step near the top end to support the gasket 16 and sealing plate 14. The gasket 16 is compressed between the can 12 and the sealing plate 14 to seal an anode 18, a cathode 20 and electrolyte within the cell 10. The anode 18, cathode 20 and a separator 26 are spirally wound together into an electrode assembly. The cathode 20 has a metal current collector 22, which extends from the top end of the electrode assembly and is connected to the inner surface of the sealing plate 14 with a contact spring 24. The anode 18 is electrically connected to the inner surface of the can 12 by a metal lead (or tab) 36 (FIG. 2). The lead 36 is fastened to the anode 18, extends from the bottom of the electrode assembly, is folded across the bottom and up along the side of the electrode assembly. The lead 36 makes pressure contact with the inner surface of the side wall of the can 12. After the electrode assembly is wound, it can be held together before insertion by tooling in the manufacturing process, or the outer end of material (e.g., separator or polymer film outer wrap 38) can be fastened down, by heat sealing, gluing or taping, for example. An insulating cone 46 is located around the peripheral portion of the top of the electrode assembly to prevent the cathode current collector 22 from making contact with the can 12, and contact between the bottom edge of the cathode 20 and the bottom of the can 12 is prevented by the inward-folded extension of the separator 26 and an electrically insulating bottom disc 44 positioned in the bottom of the can 12. Cell 10 has a separate positive terminal cover 40, which is held in place by the inwardly crimped top edge of the can 12 and the gasket 16 and has one or more vent apertures (not shown). The can 12 serves as the negative contact terminal. An insulating jacket, such as an adhesive label 48, can be applied to the side wall of the can 12. Disposed between the peripheral flange of the terminal cover 40 and the sealing plate 14 is a positive temperature coefficient (PTC) device 42 that substantially limits the flow of current under abusive electrical conditions. Cell 10 also includes a pressure relief vent. The cell sealing plate 14 has an aperture comprising an inward projecting central vent well 28 with a vent hole 30 in the bottom of the well 28. The aperture is sealed by a vent ball 32 and a thin-walled thermoplastic bushing 34, which is compressed between the vertical wall of the vent well 28 and the periphery of the vent ball 32. When the cell internal pressure exceeds a predetermined level, the vent ball 32, or both the ball 32 and bushing 34, is forced out of the aperture to release pressurized gases from the cell 10.
Examples of Li/FeS2 cells each having a pressure relief vent, a PTC device and a thermally responsive shape memory alloy current interrupter are disclosed in U.S. Pat. Nos. 4,975,341 and 4,855,195, both of which are incorporated herein by reference. Disadvantages of cells containing PTC devices can include increasing cell internal resistance with increasing temperature before operation of the PTC, an increase in internal resistance after the PTC initially operates and then resets (returns to a “normal” resistance), and excessive time for the PTC to cool and reset after the heating source is removed.
In view of the above, an object of the present invention is to provide a battery that is safe even under abusive conditions, is easy and economical to manufacture, can completely break the internal electrical circuit to help avoid excessive pressure buildup and unnecessary opening of the pressure relief vent, and has good electrical characteristics even under heavy discharge conditions. It is also desirable for the break in the circuit to be rapidly and completely reversible, without a significant increase in internal resistance.